1. Field of the Invention
The invention relates to a particle beam therapy system with an accelerator for generating a beam of charged particles, with a beam guiding unit which guides the beam from the accelerator to a treatment location with at least one beam property monitor arranged along the beam path, and one control unit communicating with the beam property monitor and the beam guiding unit, which determines the deviations of the actual properties from the desired properties of the beam and transmits control signals on the basis of these deviations for adaptation of the beam properties at the beam guiding unit. The invention further relates to a method for guiding a beam of charged particles in a particle beam therapy system.
2. Description of the Related Art
The beam therapy with charged particles, e.g. ions in the form of protons, has considerable advantages over the commonly used beam therapy with photons for patients with certain types of cancers. Irradiation with ions, especially protons, is particularly advantageous since these ions show their maximum ionization strength, and thus their maximum destructive power for tumor cells for instance, only at the end of their path to the tissues to be irradiated, the so-called Bragg peak. The effect on the healthy tissue located off the tissue to be treated and located in the path through which the beam passes can thus be reduced.
A particularly important and complex component of the magneto-optic transport system for guiding the particles from the accelerator to the patient is the carrier frame called the gantry. This carrier frame can be rotated around a rotation axis, preferably horizontal, thus enabling irradiation of the tumor tissue at various angles within a large range of up to about 360° C. The position of the huge beam guiding magnets in the field of gravity changes when the gantry is rotated. This results in a change in the gravitational force as well which can lead to an unforeseeable position deviation of the beam guiding magnets from the desired position (i.e. nominal position). As a result, there are considerable spatial deviations in the centre of the particle beam from the desired position which can lead to momentum deviations along the beam path.
Successful treatment of tumors with a particle beam, e.g. using the raster scan therapy method, necessitates accurate knowledge of beam properties, especially the beam position and the beam momentum, at the treatment location, i.e. the isocenter. Even relatively minor position deviations of multi-pole magnets of the beam guiding system can lead to undesired position deviations of the particle beam at the isocenter. These spatial deviations of the particle beam can be much larger than the position deviations of the multi-pole magnets.
For instance, a rotation-dependent shift of one millimeter of a quadrupole magnet of the gantry which is used as a focusing element can for instance lead to a spatial deviation of up to ten millimeters for the beam at the isocenter. It is difficult to completely prevent deviations (outside the required tolerance) of the beam guiding magnets when rotating the gantry using rigid mounting methods. A correction is thus required to prevent inaccuracies in the treatment.
As known from the patent specification EP 1 348 465 B1, beam deviations should be compensated using correction magnets in the beam guiding system. However, the disadvantage of the known systems is that monitors are used for recording the actual position (i.e. current position) of the beam, which influence the particle beam such that the transmission efficiency of the beam from the accelerator to the treatment location is reduced to a value that is unfavorable for the treatment. The actual position of the beam must thus be adapted to the desired position before the actual treatment. In particle beam therapy systems that are operated using the raster scan therapy method, this can lead to a significant amount of adjustment time.
In addition to the rotation-dependent influences, it can also lead to unforeseeable changes in the beam position if, during the raster scan therapy method, the beam energy is changed in the course of the treatment for instance in order to change the energy input into the tissue to be treated and thus, the penetration depth of the beam. Unforeseeable changes in the beam position can for instance be caused due to hysteresis phenomenon when changing the energizing current in the magnetic field coils of the electromagnets of the beam guiding unit. Current fluctuations in the power supplies of the magnets can also result in unforeseeable variations in the field of the beam guiding magnets.